A new study from the Hebrew University of Jerusalem challenges that assumption. Using ultra-sensitive tunneling spectroscopy, researchers probed the superconducting energy spectrum with high resolution, searching for subtle deviations from the expected single-gap behavior. The measurements revealed features that could not be reconciled with conventional single-order descriptions.

Instead, the data point to an unexpected mechanism: the materials host two superconducting states that are strongly interacting. Rather than acting independently, these two orders become coupled so effectively that their combined signatures mimic what looks like one clean superconducting gap. In other words, the system “disguises” its complexity, producing an experimental fingerprint similar to a simpler superconductor.

The team applied a more sophisticated two-band superconductivity framework to match both the detailed spectral line shape and how the superconducting state evolves in the presence of magnetic fields. This modeling clarified why earlier theories struggled to reproduce the full form of the energy spectrum, even when they captured some aspects of the response.

The results were consistent across both compounds, suggesting the phenomenon is not an isolated anomaly of NbSe₂. In TaS₂, the same hidden two-order structure emerges, implying a broader principle for superconductivity in certain layered transition-metal dichalcogenides.

The study also raises a tantalizing implication for the bulk form of NbSe₂. While the thin samples show two coupled superconducting orders, the thicker material may contain three interacting superconducting orders, implying an even richer hierarchy of electronic states beyond the minimal two-gap picture.

Beyond resolving a long-standing puzzle, the work provides a practical guide for interpreting superconducting spectra in materials where multiple orders can masquerade as one. This matters for the design of future superconducting devices, especially in regimes where precise control of electronic behavior is crucial.

As superconductivity moves toward applications in quantum technologies and ultra-efficient electronics, the ability to accurately identify hidden order parameters could improve material engineering. Understanding what electrons actually “do” inside these compounds may be the next step toward building superconductors with predictable, tunable performance.

Subject of Research:

Article Title: Two-Band Superconductivity in Few-Layer NbSe₂ and TaS₂

News Publication Date:

Web References: http://dx.doi.org/10.1103/p836-tdgw

References:

Image Credits: Avigail Ben Eliyahu

Keywords: superconductivity, quantum matter, condensed matter physics, superconducting energy gap, spectroscopy, tunneling spectroscopy, NbSe₂, TaS₂, two-band superconductivity, quantum computing

Hydrogen sulfide, a simple molecule when composed as H₃S under high pressures, made headlines several years ago owing to its remarkable superconducting transition temperature (T_c) exceeding 200 K. This stunning discovery created a paradigm shift by illustrating that conventional electron-phonon mechanisms, long thought incapable of producing superconductivity at such formidable scales, might indeed underlie these unprecedented critical temperatures. However, despite theoretical predictions and indirect experimental indications, the direct spectroscopic characterization of the superconducting gap — a hallmark of the ordered state — remained elusive until now.

The superconductor’s energy gap, often dubbed the order parameter, embodies the energy scale at which electrons pair up to form Cooper pairs and condense into a superconducting phase. Detecting and measuring this gap with high precision stands as a cornerstone to confirming the nature of superconductivity, distinguishing between conventional phonon-mediated mechanisms and more exotic pairing scenarios such as those involving spin fluctuations or unconventional symmetries. In the case of hydrogen sulfide, tunneling spectroscopy, which probes electronic states near the Fermi level with exquisite energy resolution, has successfully captured this spectral fingerprint for the first time.

According to the recent report by Du, Drozdov, Minkov, and collaborators, tunneling measurements revealed a superconducting gap value of approximately 30 millielectronvolts (meV) for the H₃S sample designated as S1. This value, although substantial, intriguingly falls below the magnitude anticipated by current theoretical frameworks, which had predicted larger gap amplitudes based on strong electron-phonon coupling models. Additionally, the so-called 2Δ/k_BT_c ratio — a dimensionless parameter that scales the gap with respect to the critical temperature — was measured at 3.54, aligning closely with the classic Bardeen-Cooper-Schrieffer (BCS) theory expectation for weak to moderate coupling superconductors.

This apparent discrepancy between empirical data and theoretical predictions opens a compelling dialogue within the superconductivity community. While the isotope effect observed, involving substitution with deuterium to form D₃S, and the consistency with an s-wave symmetry gap strongly point towards phonon-driven pairing interactions, the reduced gap magnitude invites more nuanced scrutiny. This could suggest that current theoretical treatments, though sophisticated, might yet lack the full complexity of the actual interactions or structural inhomogeneities present in these pressurized samples.

Moreover, the investigations uncovered evidence of a multigap superconducting scenario within inhomogeneous hydrogen sulfide specimens. Multigap superconductivity, wherein distinct gaps coexist on different parts of the Fermi surface or in spatially segregated superconducting phases, represents a richer and more intricate state than single-gap models. Its presence here highlights the heterogeneous nature of high-pressure hydrides and the necessity for comprehensive studies addressing phase separation, crystal structure variations, and their influence on superconducting parameters.

The implications of these findings extend far beyond hydrogen sulfide alone. The successful application of tunneling spectroscopy to dissect the superconducting gap in such challenging experimental conditions underscores the technique’s potency as a diagnostic tool. It paves the way for analogous explorations across the broader family of metal superhydrides and related materials. Understanding their superconducting gap structures with high fidelity is vital for decoding the mechanisms responsible for their often remarkably high critical temperatures and for guiding the discovery of new compounds operable at lower pressures.

These developments dovetail with a wider scientific pursuit to link microscopic interactions with macroscopic superconducting properties. Resolving the nature of electron-phonon coupling strength, anisotropies in the order parameter, or the presence of competing phases can decisively inform the direction of theoretical modeling and synthetic efforts. Furthermore, such insight contributes to the ultimate goal of engineering materials capable of attaining room-temperature superconductivity under ambient or technologically feasible conditions.

The interplay between theory and experiment witnessed in this work exemplifies the dynamic evolution of superconductivity research. The direct experimental detection of a superconducting gap, particularly with high resolution and under multi-megabar pressures, sets a new benchmark. Yet it also raises new questions, such as the precise origin of the discrepancy between observed and predicted gap magnitudes and the full character of the multigap phenomena manifesting in these complex hydride systems.

Expanding the scope of this research to include systematic isotopic substitution and pressure-dependent studies could unravel further subtleties governing pairing interactions. Additionally, complementary spectroscopic techniques like angle-resolved photoemission or neutron scattering may illuminate collective excitations and electronic structure modifications concomitant with superconductivity. Such multi-pronged approaches will build a more comprehensive picture of these enigmatic states.

In sum, the first unambiguous tunneling spectroscopic identification of the superconducting gap in hydrogen sulfide marks a seminal advance in high-pressure superconductivity science. It solidifies the central role of phonon-mediated Cooper pairing in sustaining superconductivity at record-high temperatures within this material family. Simultaneously, the nuanced deviations from theoretical expectations beckon deeper inquiries into material-specific complexities and the quest for materials surpassing current performance benchmarks.

As experimental methods continue to improve and theoretical models become increasingly sophisticated, the path toward realizing superconductors functional at ambient conditions grows ever clearer. Hydrogen sulfide and its hydride cousins today exemplify the remarkable achievements borne from this synergy of experimental innovation and intellectual exploration, promising a future where lossless electrical conduction could revolutionize energy, transportation, and beyond.

Subject of Research: Superconductivity and superconducting gap characterization in high-pressure hydrogen sulfide (H₃S).

Article Title: Superconducting gap of H₃S measured by tunnelling spectroscopy.

Article References:
Du, F., Drozdov, A.P., Minkov, V.S. et al. Superconducting gap of H₃S measured by tunnelling spectroscopy. Nature (2025). https://doi.org/10.1038/s41586-025-08895-2

Image Credits: AI Generated